Interplanar magnetic exchange in CoPS$_3$

Neutron three-axis spectrometry has been used to determine the interplanar magnetic exchange parameter in the magnetic van der Waals compound CoPS$_3$. The exchange is found to be small and antiferromagnetic, estimated to be 0.020 $\pm$ 0.001 meV, which is surprising considering that the magnetic structure is correlated ferromagnetically between the ab planes. A possible explanation, involving a small anisotropy in the exchanges, is proposed. The results are discussed with reference to the other members of the transition metal-PS3 compounds.